Process for coating and hardening steel



PROCESS FOR COATENG AND HARDENING STEEL Goran August Muller, Gotene, Sweden, assignor to American Mollerizing Corporation, Beverly Hills, Calif., a corporation of Nevada No Drawing. Application March 22, 1957 Serial No. 647,757 r 6 Claims. (Cl. 148-15) My invention relates generally to the heat treatment of metals, and more particularly, to the hardening of carbon steels. The process herein described is particularly adaptable to the production of steel springs of the type which may be subject to abrasive and corrosive effects.

This application is a continuation-in-part of my copending application Serial No. 233,941, now abandoned, filed lune 27, 1951, and entitled Process for Coating and Hardening Steel, the said application, Serial No. 233,941, being, in turn, a continuation-in-part of Serial No. 41,712, filed July 30, 1948, now abandoned, and entitled Process for Treating Steel To Be Hardened."

The usual process employed in the production of relatively high carbon steel springs and other similar articles made of hardened steel is to heat the same to a temperature somewhat higher than the critical temperature above which austenite is stable (possibly forming the steel stock into its desired shape while at high temperature), and thereafter quenching the same immediately in water, oil or some other suitable liquid.

The critical temperature for austenite formation in plain carbon steels is 1330 F. or higher, depending on the carbon content. If the rate of decrease of temperature during quenching isgreater than a certain critical rate, the soft austenite will be transformed into a very hard substance known as martensite. If the rate of decrease of temperature during quenching is less than the critical rate, the soft sustenite will be transformed into softer substances, such as pearlite.

The critical cooling rate must be exceeded in every section of the specimen if a substantially martensitic formation is to be obtained, this method being the only known method of producing a substantially martensitic structure.

Severe quenching as just described not only increases the hardness of the steel but also raises the elastic limit and tensile strength and reduces the ductility, in addition to causing high stresses in the quenched metal. To reduce the brittleness and increase the toughness of the martensitic steel it is customarily tempered for a suitable time at a temperature below that at which austenite is formed. Subsequent cooling by any manner whatsoever to room temperature finishes the heat treatment.

In the practice of this general method of producing hardened steel objects, a great many failures occur in the quenching process, because of the severe strains produced and also because of cracking at points of high stress concentration. I have found that the quenching stress concentrations and cracks in steels, especially highcarbon steels, are in a large part due to the fact that the heat is not drawn from the entire surface of the body of the heated steel at a uniform rate. Some processes in the past have attempted to obviate the quenching strains and cracks by coating steel with a metal of higher heat conductivity in order to thereby attempt to regulate the uniformity of heat transfer from hot steel during the quenching process. However, to my knowledge, such States Patent Pic processes have not solved the problem since they have overlooked an important factor, namely that the rapidity and uniformity of heat transfer from within the steel is regulated by the rate of heat transfer across the interface between the steel and the coating metal. In such prior processes, the presence of foreign matter, most frequently the iron oxides, causes a heat barrier to be set up at the interface, the heat barrier opposing the main objective of rapid uniform heat transfer from the steel.

Furthermore, the oxidation and scaling of the steel surfaces, which occurs during the usual, type of heating process, has a tendency to develop small pits and other imperfections in the steel surface which may start cracks when the material is quenched.

I have found that if the surface of the steel is protected from the action of the air or other oxidizing atmosphere during, the period of time when it is hot and when it is being heated, and if a means can be provided to draw the heat from all parts of the surface of the steel body at a relatively uniform rapid rate, and without the presence of heat barriers of any sort, a rapid quench can be employed without production of excessive strains and cracks such as have been encountered heretofore.

Bearing in mind the foregoing facts, it is a major object of the present invention to provide a process for the hardening of steel to form a substantially martensitic structure throughout the steel in which the tendency for formation of excessive strains and cracks is minimized, upon sudden quenching, as above described.

Another object of the present invention is to provide in the process of hardening steel, a mode of treatment which protects the surface of the steel against oxidation and. scaling and provides for the uniform removal of heat from the surface of the metal during quenching.

It is another object of the present invention to provide an improved process for the hardening of steel in which a dissimilar metal, having a high heat conductivity, is coated on the steel in such a manner that the coating metal-steel interface obtained is substantially free of any foreign matter, whereby to enable rapid and uniform heat transfer from said steel to occur during the quenching process.

It is important that, immediately after the coating of the steel, the quenching occur, for otherwise the critical cooling rate will not be exceeded and the steel will not be 'sufi'iciently hardened. It is also imperative to immediately quench the steel after coating so that as much alloying action as possible between the steel and the coating metal (which usually occurs rapidly at high temperature) is prevented for it is well established that the interfacial alloy layer is extremely brittle and causes the coating itself to be brittle, if a sufiicient interfacial alloy layer is formed.

It is therefore another major object of this invention to provide a process for making a hardened steel metallurgically coated to a corrosion resistant coating metal wherein the interfacial alloy layer between the steel and the coating metal is minimized so that the hardened steel thus produced has a ductile coating bonded thereto.

It is a further object of this invention to provide a process for producing an aluminum coated steel wherein the coating is chemically bonded to the surface of an oxide-free steel object and the alloy interface between the aluminum and the steel is sutficiently thin to permit flexing or other elastic deformation of the finished metal without rupture of the corrosion-resistant coating or the steel-tocoating metal bond, said process including a quenching step immediately after the coating step.

Yet another object of the present invention is to provide a process for hardening steel wherein the steel is cleaned, deoxidized i. e. freed of surface oxides, and heated in a molten salt bath, the salt that adheres to the steel being automatically removed by a subsequent coating step so as to enable quenching of the steel to proceed without the formation of a salt heat barrier, and without the necessity of an additional salt-removing step.

The foregoing and additional objects and advantages of the invention will appear from the following detailed description of a preferred embodiment thereof.

In general, the present process consists in immersing the high carbon steel stock, of which a spring orsimilar article is to be formed, in a bath of molten fluxing salt to heat the same to a temperature above the critical range. Thereafter, the stock is withdrawn into a bath of molten aluminum without any intervening oxidation to leave a coating of aluminum on the steel in close, uniform and intimate heat transfer contacttherewith. The aluminized stock is then immediately quenched in a bath of either oil or water, the bath being sufiiciently large in volume to permitcomplete cooling of the stock without raising the temperature of the quenching bath above the desired value, such as 212 F.

A particularly advantageous method of coating the steel is disclosed and claimed in my prior Patent No. 2,315,725, issued April 6, 1943, and entitled Processs for Metallization Especially Aluminization of Iron Articles. The method disclosed in said patent includes thefioatingof a molten aluminum or aluminum alloy bath on the top of a molten salt bath and the passing of the article to be coated from the salt bath directly into the overlying aluminum bath, the salt thus first acting as a flux and. thoroughly cleaning and de-oxidizing the surface of the steel article to be coated, the article being thereafter passed through the aluminum.

The salt bath is composed primarily of halides, especially the halides of the alkali and alkaline earth metals, and fluoride salts such as cryolite. It is essential that the surface of the steel be chemically clean, i. e., free of all surface oxides, and other foreign matter before the aluminum is applied, if the cooling strains and cracks are to be minimized, as previously mentioned. Fluxing salt bath compositions found especially advantageous for the purpose of completely de-oxidizing the steel are:

Fluorides may be dispensed with entirely if the base steel is relatively clean prior to its immersion in the salt bath. After the steel piece has been immersed in such a salt bath for a suitable time, the time of immersion'being dictated mainly by the temperature desired in the steel and by the degree of cleaning desired and other factors, it is withdrawn into and through an overlying molten aluminum layer as described, to be thereby coated.

The resulting tightly adherent coating of aluminum, an excellent conductor of heat, is everywhere in contact with the steel surface and nowhere insulated by oxidation products, such as scale, or pitted by the formation of scale. Thus, quenching, even though it be severe, takes place in a uniform manner, and high local stress concentrations caused by surface irregularities, such as oxides, are minimized. The ultimate result is that a hardened steel is produced without excessive strains and cracks.

it can be seen from the relative values of the thermal conductivities of aluminum, high carbon steel, and iron oxides that the greatest amount of heat transfer, will occur when aluminum is intimately and uniformly 4 bonded directly to the steel without the presence of an intermediate layer of foreign matter, such as iron oxides. The relative average value of the thermal conductivity, in the temperature ranges encountered, is for aluminum, 22 for 1% carbon steel, and about 0.2 for iron oxides. Since the conductivity of the oxides is about 700 times less than the conductivity through the steel, it can be seen that a heat barrier is interposed by the presence of the oxide. Since, according to the usual process, the oxide is generally not of uniform thickness and may, indeed, be absent in certain areas, heat transfer from the steel will occur much more rapidly at some points than at others, setting up local stress concentrations and, of course, cracks and cooling strains.

By eliminating the'last traces of oxides, as described, the problem caused by non-uniform cooling is substantially avoided, and by then coating the steel with aluminum in the manner described, to produce a chemical alloy bonding between the steel and aluminum, the much higher heat conductivity of the aluminum is utilized to its greatest extent to enable the heat to be transferred from the steel. The possibility of a heat barrier being formed within or at the surface of the steel core during the quenching process is thus substantially eliminated.

It is essential too that the steel be heated to a temperature above the austenitic temperature, that is to say, above 1333 F., theoretically, and above 1400 F. in practice. As has been described, the first function of the salt bath is to chemically clean and de-oxidize the steel. A second function of the, molten salt bath is to heat the steel. above 1400 F. prior to the aluminizing step, the salt bath being thus preferably maintained at a temperature between 1400 and 1650 F. substantially higher than the temperature contemplated in the process of the Moller patent. The time of immersion of the steel in the salt bath is, of course, dependent on the size and geometry of the steel piece, and the temperature of the salt bath.

The above-described method of hardening steel has another important advantage, viz., the ability of the steel to bond to the aluminum in a short time, during the coating operation, so as to form a relatively thin interface and ductilecoating.

The alloying reaction between steel and molten aluminum is rapid and if the two metals are allowed to come in contact for more than a few seconds at high temperatures, as in the usual process, the iron-aluminum compound, or alloy, produced by this reaction accumulates as a relatively thick interlayer. Because this compound is very brittle, a brittle non-ductile coated steel is produced. The method herein employed avoids long contact times at high temperatures by heating the steel in the salt, drawing it very quickly through the aluminum, and immediately lowering the temperature by quenching. The combination of a ductile corrosion-resistant coating, with a hardened steel core, substantially free from cooling strains and cracks, produced by the abovedescribed method, is admirably suited for the usual spring steel applications.

Experiments have been conducted to ascertain as precisely as possible the advantages of my process, as above described, contrasted with a process in which the steel is thoroughly physically cleaned, as by sandblasting, then dipped cold into molten aluminum to be coated thereby, and quenched immediately after the coating step. The results showed that the product produced according to my process, herein described, is superior, both from the point of view of producing a more ductile coating, and from the point of view of producing a steel freer from cracks and strains. The theoretical considerations heretofore described are thus proved in practice.

Attention is directed to the fact that as the steel is withdrawn from the salt bath into the overlying aluminum layer, the passage of the steel therethrough automatically accomplishes the removal of the adhering salt from the steel without the need of an additional step. It canbe seen that if the salt were not removed from the steel, it would provide an effective heat barrier (the thermal conductivity of the salt bath being 0.4 B. t. u./hr.- ft F./ft.)) to the transfer of heat in the subsequent quenching step. If, for example, the object should be drawn upwardly through a molten salt bath, after being coated, so that the adhering salt would'not be automatically removed upon withdrawal as in my process, the salt, having an extremely low thermal conductivity, will prevent rapid transfer of heat from the coated steel. Such a process results in a greater alloying action along with the consequent production of a brittle coating due to the maintenance of a higher temperature for longer periods of time after coating than in my process. Also, the presence of the insulating salt heat barrier frequently prevents the formation of a substantially martensitic steel structure because the critical cooling rate for steel cannot be readily exceeded. The foregoing observations have been tested and confirmed by actual experiments.

Attention is also directed to the fact that the quenching bath must be of suificient volume as to rapidly reduce the temperature of the hot coated steel, without itself being raised to a critical temperature beyond that necessary for the formation of martensite. For example, steel must be quenched to about 200 F. if complete martensite is to be obtained, and 0.6% carbon steel must be quenched to below 120 F. Thus, mere spraying of the steel with water or the like to remove adhering salt, for example is not a true quenching step since the steel cannot be reduced to below the critical temperature as rapidly as is necessary to exceed the critical quenching rate.

Not only does the foregoing method permit the production of steel springs and similar articles With a greatly decreased percentage of rejects due to cracks and other imperfections caused in the quenching operation, but the coating of aluminum remaining on the article forms a highly effective protection against corrosion, and is also an excellent oil-retaining surface, thus providing for the emcient lubrication of such devices as built-up multiple leaf springs.

While the process herein described is fully capable of achieving the objects and providing the advantages hereinbefore stated, it will be realized that some variations are possible without departure from the spirit of the invention. For this reason, I do not mean to be limited to the foregoing illustrative embodiment of the in vention, but rather to the scope of the appended claims.

I claim:

1. A process for producing a coated high carbon spring steel having dimensions such that it may be rapidly quenched to form a substantially martensite structure which comprises: passing steel stock into a molten salt bath to chemically flux said steel, said molten salt bath having a temperature above the critical temperature of said steel, the rate of passage through said salt bath being such that the steel attains a temperature above its critical temperature and is free of surface oxides before it is Withdrawn from said molten salt bath; withdrawing said chemically clean steel from said salt bath directly into and through a molten aluminum layer floating on at least a portion of said molten salt bath, thereby forming a chemically bonded layer of aluminum on said steel; and immediately passing'the coated steel, at a temperature above its critical temperature, into a liquid quenching bath to produce a substantially martensitic structure in said steel.

The process defined in claim 1 in which the liquid quenching bath is of suflicient volume, and is maintained at a temperature suificiently low, to rapidly reduce the temperature of said steel to below 200 F.

3. A process for producing a coated high carbon spring steel having dimensions such that it may be rapidly quenched to form a substantially martensitic structure throughout and having a carbon content in excess of 0.50 percent, which comprises: passing steel stock into a molten salt bath comprising metal chlorides and fluorides to chemically flux said steel, said molten salt bath having a temperature above the critical temperature of said steel and between l400 and 1650 F., the rate of passage through said salt bath being such that the steel attains a temperature above its critical temperature and is free of surface oxides before it is withdrawn from said molten salt bath; withdrawing said chemically clean steel from said salt bath directly through a molten aluminum layer floating on said molten salt bath, thereby forming a ferroaluminum bond on said oxide-free steel; and immediately passing the coated steel, at a temperature above its critical temperature, into a liquid quenching bath selected from the group consisting of water and oil to rapidly reduce the temperature to below 200 F. and produce a substantially martensitic structure in said steel.

4. A process for producing a coated high carbon spring steel having dimensions such that it may be rapidly quenched to form a substantially martensitic structure throughout and having a carbon content in excess of 0.50

percent, which comprises: passing said steel into a molten salt bath comprising metal fluorides and chlorides to chemically flux said steel, said molten salt bath having a temperature above the critical temperature of said steel and between 1400 and 1650 F., the rate of passage through said salt bath being such that the steel attains a temperature above its critical temperature and is free of surface oxides before it is withdrawn from said molten salt bath; withdrawing said chemically clean steel from said salt bath directly through a molten aluminum layer floating on said molten salt bath thereby forming an oxide-free ferro-aluminum interface on said steel, the heat transfer across said steel being controlled by the rate of heat transfer across said ferro-aluminum interface; and immediately passing the aluminum-coated steel, at a temperature above its critical temperature, into a liquid bath selected from the group consisting of water and oil to rapidly reduce the temperature to below 200 F., and produce a substantially martensitic structure in the steel.

5. A process for producing a coated high carbon spring steel having dimensions such that it may be rapidly quenched to form a substantially martensitic structure throughout, by heating said steel above its critical temperature and immediately quenching said steel to a temperature at which martensite forms substantially without development of scales or cracks induced by such rapid quenching, comprising the steps of; passing said steel into a molten salt bath comprising metal chlorides and fluorides to chemically flux said steel, said molten salt bath having a temperature above the critical temperature of said steel and between l400 and 1650 F., the rate of passage through said salt bath being such that the steel attains a temperature above its critical temperature and is free of surface oxides before it is withdrawn from said molten salt bath; withdrawing said chemically clean steel from said salt bath directly through a molten aluminum layer floating on said molten salt bath thereby forming an aluminum-coated steel having an oxide-free ferro-aluminum interface, the heat transfer across said steel being controlled by the rate of heat transfer across said ferro-aluminum interface; and immediately passing said aluminum-coated steel, at a temperature above its critical temperature, into a liquid bath selected from the group consisting of water and oil to rapidly reduce the temperature to below 200 F., and produce a substantially martensitic structure in said steel, said quenching step causing an intermediate freezing of said intimately bonded aluminum onto said steel.

6. A process for producing a coated high carbon spring steel having dimensions such that it may be rapidly quenched to form a substantially martensitic structure throughout and having a carbon content in excess of 0.50 percent, which comprises: passing steel stock into a molten salt bath comprising metal chlorides to chemically flux said steel, said molten salt bath having a temperature above the critical temperature of said steel and between 1400 and 1650 F., the rate of passage through said salt bath being such that the steel attains a temperature above its critical temperature and is free of surface oxides before it is withdrawn from said molten salt bath; withdrawing said chemically clean steel from said salt bath directly through a molten aluminum layer floating on said molten salt bath, thereby forming a ferro-aluminum bond 8 on said oxide-free steel; and immediately passing the coated steel, at a temperature above its critical temperature, into a liquid quenching bath selected from the group consisting of water and oil to rapidly reduce the temperature to below 200 F. and produce a substantially martensitic structure in said steel.

References Cited in the file of this patent UNITED STATES PATENTS 2,315,725 Moller Apr. 6, 1943 2,396,730 Whitfield Mar. 19, 1946 2,569,097 Grange et al Sept. 25, 1951 

1. A PROCESS FOR PRODUCING A COATED HIGH CARBON SPRING STEEL HAVING DIMENSIONS SUCH THAT IT MAY BE RAPIDLY QUENCHED TO FORM A SUBSTANTIALLY MARTENSITE STRUCTURE WHICH COMPRISES: PASSING STEEL STOCK INTO A MOLTEN SALT BATH TO CHEMICALLY FLUX SAID STEEL, SAID MOLTEN SALT BATH HAVING A TEMPERATURE ABOVE THE CRITICAL TEMPERATURE OF SAID STEEL, THE RATE OF PASSAGE THROUGH SAID SALT BATH BEING SUCH THAT THE STEEL ATTAINS A TEMPERATURE ABOVE ITS CRITICAL TEMPERATURE AND IS FREE OF SURFACE OXIDES BEFORE IT IS WITHDRAWN FROM SAID MOLTEN SALT BATH; WITHDRAWING SAID CHEMICALLY CLEAN STEEL FROM SAID SALT BATH DIRECTLY INTO AND THROUGH A MOLTEN ALUMINUM LYER FLOATING ON AT LEAST A PORTION OF SAID MOLTEN SALT BATH, THEREBY FORMING A CHEMICALLY BONDED LAYER OF ALUMINUM ON SAID STEEL AND IMMEDIATELY PASSING THE COATED STEEL, AT A TEMPERATURE ABOVE ITS CRITICAL TEMPERATURE, INTO A LIQUID QUENCHING BATH TO PRODUCE A SUBSTANTIALLY MARTENSITIC STRUCTURE IN SAID STEEL. 